EQUITY METHOD INVESTMENTS - RELATED PARTY TRANSACTIONS (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
ATC
Related party transactions
Charges to equity method investee for services, construction, and operations	$ 9.9	$ 11.3	$ 12.5
Total costs for network transmission services provided by ATC	103.8	104.9	100.3
WRPC
Related party transactions
Charges to equity method investee for services, construction, and operations	1.4	0.9	0.8
Purchases of energy from WRPC	3.7	3.7	5.0
Net proceeds from WRPC sales of energy to MISO	0	0	2.9
Number of hydroelectric plants owned	2
Portion of energy output sold to WPS	50.00%
Portion of energy output sold to Wisconsin Power and Light	50.00%
AMP Trillium, LLC
Related party transactions
Notes receivable due from related party	13.8
Net proceeds from the sale of CNG fueling stations	$ 3.1

Number of hydroelectric plants operated.

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Portion of the energy output of the plants sold to the entity.

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Portion of the energy output of the plants sold to a party other than the entity.

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Revenues, net of expenses, associated with the sale of compressed natural gas fueling stations to related parties.

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Cash received from the sales made by equity method investees during the current period.

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Reflects charges to related parties for services and/or construction (excluding transactions that are eliminated in consolidated or combined financial statements) during the period.

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For an unclassified balance sheet, amounts due from parties associated with the reporting entity as evidenced by a written promise to pay.

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Expenses recognized resulting from transactions (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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Purchases during the period (excluding transactions that are eliminated in consolidated or combined financial statements) with related party.

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